Microwave plasmas are used in the industrial chemical processing of gases. This is typically accomplished by flowing the gases to be reacted through an elongated vessel while microwave radiation is coupled into the vessel to generate a plasma. The plasma cracks the gas molecules into component species. Microwave chemical processing systems are effective because microwave plasmas operate at relatively high power coupling efficiencies at low ion energies, and are capable of supporting various gas reactions, such as the conversion of methane into hydrogen and carbon particulates, the conversion of carbon dioxide into oxygen and carbon, and coating particulates and other seed materials with other layers for functionalization and complex layered materials and aggregates processing.
Typical systems for chemical gas processing include a quartz reaction chamber through which process gases flow, and a microwave magnetron source coupled to the reaction chamber through a waveguide. The input microwave radiation can be continuous wave or pulsed. Systems are designed to control the effective coupling of the microwave radiation into the reaction chamber, and the gas flow within the reaction chamber to improve the energy absorption by the flowing gas. Often the systems include a wedge located where the microwave waveguide intersects the quartz reaction chamber, to concentrate the electric field within a small area, and the waveguide conductive walls are not exposed to the gases to be processed.
One example of chemical processing is the microwave processing of methane to produce hydrogen. Methane can be cracked by a plasma into CHx radicals and H-atoms. When such systems are operated in continuous mode, the H-atom density is mainly controlled by the gas temperature, which is directly related to the microwave power density, and in some cases by diffusion processes. The CHx radical density, likewise, is controlled by the gas temperature and H-atom concentrations. Alternatively, when such systems are operated in pulsed mode, H-atom and CHx radical production is controlled by in-pulse power density and its associated higher plasma kinetic energy, which controls gas temperature and thermal dissociation. Typically, during the time the plasma is off the H-atoms recombine and are consumed. Short duty cycles are used to increase the in-pulse power for a constant time-averaged power, and short off-plasma times are used to limit H-atom recombination. Therefore, pulsed systems crack the methane into hydrogen and other hydrocarbon radicals more efficiently (i.e., using less time-averaged input power) than continuous wave systems.